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All future respins will cc. This series changes mremap() semantics (I will update the manpage accordingly of course). Cheers, Lorenzo On Mon, Jul 07, 2025 at 06:27:43AM +0100, Lorenzo Stoakes wrote: > Historically we've made it a uAPI requirement that mremap() may only > operate on a single VMA at a time. > > For instances where VMAs need to be resized, this makes sense, as it > becomes very difficult to determine what a user actually wants should they > indicate a desire to expand or shrink the size of multiple VMAs (truncate? > Adjust sizes individually? Some other strategy?). > > However, in instances where a user is moving VMAs, it is restrictive to > disallow this. > > This is especially the case when anonymous mapping remap may or may not be > mergeable depending on whether VMAs have or have not been faulted due to > anon_vma assignment and folio index alignment with vma->vm_pgoff. > > Often this can result in surprising impact where a moved region is faulted, > then moved back and a user fails to observe a merge from otherwise > compatible, adjacent VMAs. > > This change allows such cases to work without the user having to be > cognizant of whether a prior mremap() move or other VMA operations has > resulted in VMA fragmentation. > > In order to do this, this series performs a large amount of refactoring, > most pertinently - grouping sanity checks together, separately those that > check input parameters and those relating to VMAs. > > we also simplify the post-mmap lock drop processing for uffd and mlock()'d > VMAs. > > With this done, we can then fairly straightforwardly implement this > functionality. > > This works exclusively for mremap() invocations which specify > MREMAP_FIXED. It is not compatible with VMAs which use userfaultfd, as the > notification of the userland fault handler would require us to drop the > mmap lock. > > The input and output addresses ranges must not overlap. We carefully > account for moves which would result in VMA merges or would otherwise > result in VMA iterator invalidation. > > Lorenzo Stoakes (10): > mm/mremap: perform some simple cleanups > mm/mremap: refactor initial parameter sanity checks > mm/mremap: put VMA check and prep logic into helper function > mm/mremap: cleanup post-processing stage of mremap > mm/mremap: use an explicit uffd failure path for mremap > mm/mremap: check remap conditions earlier > mm/mremap: move remap_is_valid() into check_prep_vma() > mm/mremap: clean up mlock populate behaviour > mm/mremap: permit mremap() move of multiple VMAs > tools/testing/selftests: extend mremap_test to test multi-VMA mremap > > fs/userfaultfd.c | 15 +- > include/linux/userfaultfd_k.h | 1 + > mm/mremap.c | 502 ++++++++++++++--------- > tools/testing/selftests/mm/mremap_test.c | 145 ++++++- > 4 files changed, 462 insertions(+), 201 deletions(-) > > -- > 2.50.0